The present invention relates to heat-shrinkable polyester films, and more particularly, it relates to heat-shrinkable polyester films suitable for full labels on bottles, particularly full labels on PET bottles, which exhibit, when used for full labels on bottles, good shrinkage finish involving only rare occurrence of wrinkles, shrinkage spots, strains and shrinkage failure during heat shrinkage, and which exhibit a low decrease in low temperature shrinkability after a lapse of time.
For heat-shrinkable films, particularly used as labels on the barrels of bottles, films made of polyvinyl chloride or polystyrene have mainly been employed. However, polyvinyl chloride films have recently raised a serious problem on the evolution of chlorine-containing gases in the incineration for disposal, whereas polystyrene films have a serious problem on the difficulty of printing. Further, in the collection and recycling of PET bottles, labels made of resins other than PET, such as polyvinyl chloride or polystyrene, should be separated from the bottles. Therefore, considerable attention has been drawn to heat-shrinkable polyester films having no such problems.
In recent years, since colored bottles are not suitable for regeneration in regard to the recycling of PET bottles, alternative plans have been studied. Among them is a method of employing colored bottles and allowing colored labels to shrink over these bottles.
However, when used for full labels on bottles, since these bottles have complicated shapes in great variety, the conventional heat-shrinkable films may cause a serious problem on the shrinkage finish. In particular, for full labels on narrow-mouthed bottles, such as beverage bottles, having a great difference in diameter between their barrels and mouths, the conventional heat-shrinkable films may cause shrinkage failure at the necks of the bottles. Therefore, heat-shrinkable films used for full labels on such bottles should have excellent heat-shrinkage characteristics including high shrinkability. Further, the conventional heat-shrinkable films may require a change in the conditions of shrinkage finish by a decrease in low temperature shrinkability during storage before shrinkage, or they may exhibit poor shrinkage in some cases.
As described above, the conventional heat-shrinkable polyester films have insufficient performance for full labels on bottles.
The present invention, which can solve the above problems, has been made to provide heat-shrinkable polyester films suitable for full labels on bottles, particularly full labels on PET bottles, which exhibit, when used for full labels on bottles, good shrinkage finish involving only rare occurrence of wrinkles, shrinkage spots, strains and shrinkage failure during heat shrinkage, and which exhibit a low decrease in low temperature heat shrinkability after a lapse of time.
Thus the present invention provides heat-shrinkable polyester films each having: a heat shrinkability of 30-60% after treatment in hot water at 70xc2x0 C. for 5 seconds and of 65% or higher after treatment in hot water at 85xc2x0 C. for 5 seconds, both in the main shrinkage direction, and of 10% or lower after treatment in hot water at 85xc2x0 C. for 5 seconds in a direction perpendicular to the main shrinkage direction; a heat shrinkability after storage at 25xc2x0 C. for one month of 25% or higher after treatment in hot water at 70xc2x0 C. for 5 seconds in the main shrinkage direction; and a thickness distribution of 6% or lower, thereby solving the above problems.
The heat-shrinkable polyester films of the present invention are preferably produced from polyester compositions containing polyesters and polyester elastomers, which polyesters are composed of dicarboxylic acid components and diol components as constituent components. The polyester compositions preferably contain polyesters at 50-95 wt % and polyester elastomers at 5-50 wt %, more preferably at 5-20 wt %, and most preferably at 5-10 wt %. The reasons for this are that: when the amounts of polyester elastomers are higher than 50 wt %, there is a tendency for heat-shrinkable films to hardly have shrinkability required for close adhesion to vessels, which is not preferred; and when the amounts of polyester elastomers are lower than 5 wt %, heat-shrinkable films can hardly attain low temperature shrinkability and can easily cause shrinkage at a higher speed, so that there is a tendency for heat-shrinkable films to have deteriorated shrinkage finish involving the occurrence of wrinkles and strains during shrinkage, which is also not preferred.
The dicarboxylic acid components constituting the polyesters used in the present invention may include aromatic dicarboxylic acids terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid and ortho-phthalic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid; and alicyclic dicarboxylic acids.
When aliphatic dicarboxylic acids (e.g., adipic acid, sebacic acid, decanedicarboxylic acid) are contained in the polyesters, their amounts may preferably be lower than 3 mol %. For heat-shrinkable polyester films obtained from the polyesters containing these aliphatic dicarboxylic acids at 3 mol % or higher, their film stiffness in the high-speed fitting is insufficient.
Further, the polyesters preferably contain no three or more functional polycarboxylic acids (e.g., trimellitic acid, pyromellitic acid, their anhydrides). For heat-shrinkable polyester films obtained from the polyesters containing these polycarboxylic acids, their high shrinkability required can hardly be attained.
The diol components constituting the polyesters used in the present invention may include aliphatic diols such as ethylene glycol, propanediol, butanediol, neopentyl glycol and hexanediol; alicyclic diols such as 1,4-cyclohexanedimethanol; and aromatic diols.
The polyesters used in the heat-shrinkable polyester films of the present invention preferably have glass transition temperatures (Tg) adjusted to 60-75xc2x0 C. by incorporation of at least one selected from diols containing 3-6 carbon atoms (e.g., propanediol, butanediol, neopentyl glycol, hexanediol).
For heat-shrinkable polyester films exhibiting particularly excellent shrinkage finish, the use of neopentyl glycol as one of the diol components is preferred.
The polyesters preferably contain neither diols containing 8 or more carbon atoms (e.g., octanediol), nor three or more functional polyhydric alcohols (e.g., trimethylolpropane, trimethylolethane, glycerin, diglycerin). For heat-shrinkable polyester films obtained from the polymers containing these diols or polyhydric alcohols, their high shrinkability required can hardly be attained.
The polyesters preferably contain none of diethylene glycol, triethylene glycol, and polyethylene glycol, if possible. In particular, diethylene glycol may easily be formed as a by-product component in the polymerization of polyesters. The polyesters used in the present invention preferably contain diethylene glycol in amounts of smaller than 4 mol %.
When two or more polyesters are used in admixture, the amounts of acid components and the amounts of diol components are relative to the total amount of all acid components and the total amount of all diol components, respectively, both of which are contained in these polyesters, independently of whether or not transesterification has been carried out after the mixing.
To the heat-shrinkable polyester films of the present invention, there may preferably be added, for the purpose of improving their self-lubricating properties, inorganic lubricants such as titanium dioxide, fumed silica, kaolin and calcium carbonate; or organic lubricants such as long-chain fatty acid esters, and there may further be added, if necessary, additives such as stabilizers, colorants, antioxidants, defoamers, antistatic agents and ultraviolet light absorbers.
All the above polyesters can be prepared by polymerization according to the conventional methods. For example, the polyesters can be obtained using the direct esterification method in which dicarboxylic acids are directly reacted with diols or using the transesterification method in which dicarboxylic acid dimethyl esters are reacted with diols. The polymerization may be carried out in batch or continuous manner.
The polyester elastomers used in the present invention refer to polyester block copolymers composed of high melting point crystalline polyester segments (hard segments) and low melting point soft polymer segments with molecular weights of 400 or higher (soft segments) as constituent units, in which high molecular weight polymers formed only of the high melting point crystalline polyester segment constituent components have melting points of 200xc2x0 C. or higher and the melting points or softening points when measured for only the low melting point soft polymer segment constituent components are 80xc2x0 C. or lower.
The high melting point crystalline polyester segment constituent components, which have melting points of 200xc2x0 C. or higher when formed only themselves into fibrillatable high molecular weight polymers, may include, for example, polyester segments composed of aromatic dicarboxylic acid residues, such as terephthalic acid, isophthalic acid, 1,5-naphthalenedicarboxylic acid and 2,6-naphthalenedicarboxilic acid, and aliphatic, aromatic or alicyclic diol residues, such as ethylene glycol, propylene glycol, tetramethylene glycol, pentamethylene glycol, 2,2-dimethyltrimethylene glycol, hexamethylene glycol, decamethylene glycol, p-xylene glycol and cyclohexanedimethanol; polyester segments composed of oxyacid residues, such as p-(xcex2-hydroxyethoxy)benzoic acid and p-oxybenzoic acid pivalolactone; polyether ester segments composed of aromatic ether dicarboxylic acid residues, such as 1,2-bis(4,4xe2x80x2-dicarboxymethylphenoxy)ethane and di(4-carboxyphenoxy)-ethane, and the same diol residues as described above; and polyamide ester segments composed of aromatic amide dicarboxylic acid residues, such as bis(N-p-carboethoxydiphenyl)terephthalimide, and the same diol residues as described above.
There can also be mentioned copolyester segments using two or more of the above acids and/or two or more of the above glycols.
The low melting point soft polymer segments constituent components with molecular weights of 400 or higher refer to those which exhibit a substantially amorphous state among the polyester block copolymers and which have melting points or softening points of 80xc2x0 C. or lower when measured for only these segment constituent components. Their molecular weights are usually 400-8000, preferably 700-5000.
The proportions of low melting point soft polymer segment constituent components in the polyester block copolymers are preferably 1-90 wt %. Particularly preferred proportions are 5-80 wt %.
Typical examples of the low melting point soft polymer segment constituent components may include polyethers such as polyethyleneoxide glycol, polypropyleneoxide glycol, polytetramethyleneoxide glycol, copolymerized glycols of ethyleneoxide and propyleneoxide, and copolymerized glycols of ethyleneoxide and tetrahydrofuran; aliphatic polyesters such as polyneo-pentyl azelate, polyneopentyl adipate and neopentyl sebacate; polymers of cyclic esters, such as poly-xcex5-caprolactone.
The polyester elastomers are contained in the polyester resin compositions constituting polyester films, preferably at about 3-50 wt % in usual cases, more preferably at 5-30 wt %, and most preferably at 7-20 wt %.
The polyester elastomers used in the present invention are preferably polyester block copolymers using poly-xcex5-caprolactone as the low melting point polymer segment. In particular, preferred are those which contain the polybutylene terephthalate component. The use of polyester resin compositions containing such polyester elastomers leads to the advantages that the resulting heat-shrinkable polyester films can easily attain low temperature shrinkability and have an improved effect on the prevention of a decrease in shrinkability after a lapse of time.
All the above polyester elastomers can be prepared by the conventional methods known in the art. For example, the polyester elastomers can be obtained by polymerization into prescribed block copolymers using the transesterification method in which dialkyl esters such as dicarboxylic acid dimethyl esters are reacted with diols and, if necessary, cyclic esters, or using the direct esterification method in which dicarboxylic acids are directly reacted with diols and, if necessary, cyclic esters. The polymerization may be carried out in batch or continuous manner.
For the heat-shrinkable polyester films of the present invention, the heat shrinkability as calculated by the following equation from the values of side length before and after shrinkage by treatment in hot water without a load.
xe2x80x83Heat shrinkability=Side length before shrinkagexe2x88x92Side length after shrinkage/Side length before shrinkagexc3x97100(%)
should be 30-60%, preferably 40-60%, after treatment in hot water at 70xc2x0 C. for 5 seconds, and 65% or higher, preferably 75-95%, after treatment in hot water at 85xc2x0 C. for 5 seconds, both in the main shrinkage direction, and 10% or lower, preferably 6% or lower, after treatment in hot water at 85xc2x0 C. for 5 seconds in a direction perpendicular to the main shrinkage direction.
When the heat shrinkability in the main shrinkage direction is lower than 30% after treatment in hot water at 70xc2x0 C. for 5 seconds, it results in a failure of low temperature shrinkage, requiring the rise in temperature for shrinkage, which is not preferred. In contrast, when the heat shrinkability is higher than 60%, label jumping occurs by heat shrinkage, which is also not preferred.
When the heat shrinkability in the main shrinkage direction is lower than 65% after treatment in hot water at 85xc2x0 C. for 5 seconds, shrinkage failure occurs at the mouth portions of bottles depending on the bottle shapes, which is not preferred. In contrast, when the heat shrinkability is higher than 95%, there is a possibility that label jumping occur because of shrinkability remaining after heat shrinkage.
For the heat-shrinkable polyester films of the present invention, the heat shrinkability after storage at 25xc2x0 C. for one month should be 25% or higher after treatment in hot water at 70xc2x0 C. for 5 seconds in the main shrinkage direction. When the heat shrinkability after storage at 25xc2x0 C. for one month is lower than 25%, it requires a change in the conditions of shrinkage finish, or the resulting films exhibit poor shrinkage in some cases, which is not preferred.
The heat-shrinkable polyester films of the present invention are preferably, but not particularly limited to, 10 to 200 xcexcm, more preferably 20 to 100 xcexcm, in thickness as the heat-shrinkable films for labels.
For the heat-shrinkable polyester films of the present invention, the thickness distribution should be 6% or lower. When the thickness distribution is higher than 6%, the resulting films occur the deviation of patterns in the superposition of many colors when processed in the multi-color printing, which is not preferred.
The following will describe a specific example of the process for producing the heat-shrinkable polyester film of the present invention; however, the present invention is not limited to this example.
The polyester base materials used in the present invention are dried using a dryer such as hopper dryer or paddle dryer, or a vacuum dryer, and melt at a temperature of 200-300xc2x0 C. and extruded into a film shape. For extrusion, any of the existing methods can be used, such as T-die method or tubular method. After extrusion, rapid cooling gives an unstretched film.
The resulting unstretched film is then stretched at a ratio of 4.2 or higher, preferably 4.5 or higher, in the transverse direction at a temperature of Tg xc2x0C. or higher but lower than Tg+15xc2x0 C., preferably Tg+5xc2x0 C. or higher but lower than Tg+15xc2x0 C.
The stretched film is heat treated, if necessary, at a temperature of 70-100xc2x0 C. to give a heat-shrinkable polyester film.
The method of stretching may include uniaxial stretching only in the transverse direction (TD) with a tenter, in which case the film can further be stretched in the machine direction (MD) to thereby achieve biaxial stretching. Such biaxial stretching may be carried out either by the sequential biaxial stretching method or by the simultaneous biaxial stretching method, and the film may further be stretched, if necessary, in the machine or transverse direction.
For attaining the purpose of the present invention, the transverse direction (i.e., the direction perpendicular to the direction of extrusion) is practical as the main shrinkage direction; therefore, the above explanation was made for an example of the film formation when the transverse direction is taken as the main shrinkage direction. However, the film formation in which the machine direction (i.e., the direction of extrusion) is taken as the main shrinkage direction can also be carried out substantially in the same manner as described above, except that the stretching direction in the above explanation is turned 90 degrees around the line perpendicular to the film surface.
In the present invention, unstretched films obtained from polyesters are preferably stretched at a temperature of Tg xc2x0C. or higher but lower than Tg+15xc2x0 C., preferably Tg xc2x0C or higher but lower than Tg+12xc2x0 C.
When unstretched films are stretched at a temperature of lower than Tg xc2x0C., the resulting films can hardly have heat shrinkability as some of the claimed requirements of the present invention, and these films have deteriorated transparency, which is not preferred.
When unstretched films are stretched at a temperature of Tg+15xc2x0 C. or higher, the resulting films exhibit insufficient film stiffness in the high-speed fitting, and these films have remarkably deteriorated thickness distribution, which is also not preferred.
For the heat-shrinkable polyester films of the present invention, the thickness distribution of the films as calculated from the values of film thickness by the following equation:       Thickness    ⁢          xe2x80x83        ⁢    distribution    =                              Maximum          ⁢                      xe2x80x83                    ⁢          thickness                -                  Minimum          ⁢                      xe2x80x83                    ⁢          thickness                            Average        ⁢                  xe2x80x83                ⁢        thickness              xc3x97    100    ⁢          xe2x80x83        ⁢          (      %      )      
is preferably 6% or lower, more preferably 5% or lower.
The films having a thickness distribution of 6% or lower are easy to achieve the superposition of colors in the three-color printing carried out, for example, in the evaluation of shrinkage finish, whereas the films having a thickness distribution of higher than 6% are not preferred from the viewpoint of color superposition.
To make even thickness distribution in the heat-shrinkable polyester films of the present invention, unstretched films are preferably heated to a prescribed film temperature at a low air flow rate with a heat transmission coefficient of 0.0013 cal/cm2xc2x7secxc2x7xc2x0C. (0.0054 J/cm2xc2x7secxc2x7K) or lower in the step of preheating to be carried out prior to the step of stretching when unstretched films are stretched in the transverse direction with a tenter.
To prevent heat evolution in the films during stretching to reduce the unevenness of film temperature in the width direction of the films, the step of stretching is preferably carried out with a heat transmission coefficient of 0.0009 cal/cm2xc2x7secxc2x7xc2x0C. (0.0037 J/cm2xc2x7secxc2x7K) or higher, more preferably 0.0011-0.0017 cal/cm2xc2x7secxc2x7xc2x0C. (0.0045-0.0071 J/cm2xc2x7secxc2x7K).
When the heat transmission coefficient in the step of pre-heating is higher than 0.0013 cal/cm2xc2x7secxc2x7xc2x0C. (0.0054 J/cm2xc2x7secxc2x7K) or the heat transmission coefficient in the step of stretching is lower than 0.0009 cal/cm2xc2x7secxc2x7xc2x0C. (0.0037 J/cm2xc2x7secxc2x7K), the resulting films can hardly have even thickness distribution, so that these films cause the deviation of patterns in the super-position of many colors when processed in the multi-color printing, which is not preferred.